Furnace for electrolysis of aluminum operating at constant height of liquid levels



May l2, 1964 G. DE VARDA 3,133,008

F'URNACE FOR ELECTROLYSIS OF' ALUMINUM OPERATING AT CONSTANT HEIGHT OF LIQUID LEVELS Filed Jan. 28, 1958 4 Sheets-Sheet 1 May 12, 1964 G. DE vARDA 3,133,008

FURNACE FOR ELECTROLYSIS OF ALUMINUM OPERATING AT CONSTANT HEIGHT 0E LIQUID LEVELS 3,133,008 G AT G. DE VARDA TROLYSIS OF May 12, 1964 FURNACE FOR ELEC ALUMINUM OPERATIN CONSTANT HEIGHT OF' LIQUID LEVELS 4 Sheets-Sheet 3 Filed Jan. 28, 1958 May l2, 1964 G. DE vARDA 3,133,008

FURNACE FOR ELECTROLYSIS OF ALUMINUM OPERATING AT CONSTANT HEIGHT OF' LIQUID LEVELS Filed Jan. 28, 1958 4 Sheets-Sheet 4 United States Patent O FURNACE FR ELECTROLYSIS OF ALUMINUM OPERATING AT CONSTANT HEIGHT F LIQUID LEVELS Giuseppe de Varda, 8 Via San Sisto, Milan, Italy Filed Jan. 28, 1958, Ser. No. 711,577 Claims priority, application Italy Jan. 31, 1957 Claims. (Cl. 2041-244) This invention relates to an improvement in a furnace apparatus and process for electrolysis of a metal compound dissolved or suspended in a molten bath.

It particularly concerns an improvement in the production of aluminum by electrolysis of alumina or other aluminum compound, in a fused salt bath, such as cryolite, at temperatures, for example, of from about 900 C. to 1000" C. In such processes carbon electrodes are generally employed which are in part consumed in the electrolysis. It is desirable and very advantageous, to maintain all, or at least most, of the operating factors, or conditions, constant over a period of operating time. These factors may, for instance, be the total and partial voltages, the intensity and distribution of electrolysis current, the electrode spacing, the depth and composition of the bath, the characteristics and areas of the active electrode surfaces, and the temperature in the various zones of the furnace.

One factor which it has not been possible to stabilize so far, is the height or level of the metal collected during the electrolysis at the bottom of the cell. This consideration is apart from the sudden interruption or'disturbance of the normal furnace operating conditions every time the periodic operation of tapping the cell has to be performed.

In conventional furnaces employing horizontal electrode and bath layers said height increases'gradually during the electrolysis. The tapping operation, however, results, periodically in the sudden lowering of said level by some, or several, centimeters.

Ordinarily the quantity of metal removed by tapping is such as to restore said height level substantially to the same, minimum, starting value resulting after previous tappings.

In multi-cell furnaces of the kind described in G. de Varda applications Serial No. 587,985 iiled May 29, 1956, now U.S. Patent No. 2,952,592, disclosing the necklace arrangement of cells, Serial No. 551,679, filed December 7, 1955, now U.S. Patent No. 2,938,843 and others, the increment in height of the molten aluminum layer collecting in the lower chambers is ordinarily a number of decimeters. Therefore, this is also the order of magnitude for the lowering of the level of the molten aluminum layer that covers the bottom of the lower chamber for molten aluminum after every operation of tapping the cell.

In the former case, that of the conventional furnace, at least the height level of the anode must consequently be varied. This is done by means of external mechanisms provided for adjustment of the electrode spacing. This operation involves various inconveniences. Among them are the need for periodical checking and adjusting, the tendency to operate with excessive electrode spacing, the periodic oscillation or variation in temperature, and the lateral anodic corrosion.

In the second case, that' of the multicell furnaces of said copending applications, the inconveniences are prevailingly due to the type of construction. For instance there is the greater height and, therefore, increased cost of the individual cells. There are also operation diiiiculties. For example, there is the need, or at least opportunity, for operatively combining the pouring of the metal with the periodic anode restoration, if the furnace is not equipped with self-restoring devices for the anodes as described in my copending application Ser. No. 706,077, filed December 30, 1957, now U.S. Patent No. 2,959,527.

The process of the present invention surprisingly eliminates, to av great extent, the aforesaid inconveniences, since it facilitates maintenance of the height level of the molten aluminum in the cells substantially constant during electrolysis (and pouring).

A device, which also forms part of the present invention, permits the removal from the cell, automatically and expeditiously, and in a substantially continuous manner, of quantities of molten aluminum substantially equivalent to those being produced by the electrolysis.

In one important aspect, the invention resides in replacing the periodic or intermittent aluminumtapping operation by a continuous, or nearly continuous, and automatically functioning overflow of the aluminum over a Weir, or other overflow or overfall device, and preferably down into a hot receptacle or channel preferably placed, enclosed, or formed within the heat insulated interior of the furnace. The height of the Weir is predetermined so as to obtain the optimum amount of overflow of excess aluminum, coming from the bottom of the individual electrolysis gaps. V

This receptacle is otherwise completely separated from the respective cell or cells which feed it. The aluminum collected in said receptacle may be sucked, drawn, discharged, or removed `in any manner. It is always ready in the molten state, whenever needed. An important aspect is that its removal does not affect the level of the molten aluminum in the interior of the cell or cells feeding said receptacle to the last extent.

To raise the molten aluminum, that is, the molten aluminum only but not the molten bath, to the level of the overfall, means are employed which apply the principle of communicating vessels, which means are described and illustrated for the tapping pockets of my application Serial No. 587,985 disclosing a necklace-type multicell furnace.

Illustrative examples of convenient embodiments of the invention are shown in the accompanying drawings in which:`

FIG. 1 is a vertical, longitudinal view, chiefly sectional, of a number of contiguous cells of a necklace-type multicell furnace having self-restoring anodes, as modified in accordance with the present invention; the partial section is on line 1 1 of FIG. 2;

FIG. 2 comprises two partial horizontal sectional-views taken at line A-A of FIG. 1, the sections being taken respectively at a lower level through the graphite bipolar electrodes, and at a higher level through the channels which ensure circulation of the bath from gap to the next;

FIG. 3 constitutes a top view at the right and a sectional view at the left, taken at B-B of FIG. l;

FIG. 4 is a transverse cross-section of the furnace of FIG.v 1;

FIG. 5 is a longitudinal vertical section of the said furnace taken parallel to and near its longitudinal axis,- at line D-D of FIG. 4;

FIG. 6 is a vertical section of a conventional furnace having horizontal layers and two self-baking electrodes, as modified in accordance with the present invention; FIG. 6 is taken at B-B of FIG. 7;

FIG. 7 is a horizontal section of the two-electrode furnace of FIG. 6, taken at A-A of FIG. 6.

FIG. 1 is a partial section of the furnace described in my application Serial No. 587,985, modified in accordance with my application Ser. No. 706,077 and in accordance with the present invention. 4

In the drawing, the reference numeral 2 (FIG. l) represents blocks ofinert refractory materials, which form one electrolysis the top head parts of the bipolar electrodes 1. Blocks 2 rest on vertical walls 13 (FIGS. l and 2) of the same refractory. Two walls 13 divide the bipolar electrodes into three sections (FIG. 2). Blocks` 2 are provided with vertical grooves 14 and are passed through by the conduits which permit the regulated ilow of the bath through the necklace circuit of cells. Above blocks 2 are fixed layers 6 and mobile layers 8, each of heat-insulating material. On the blocks 2 rest refractory pieces 3, of chimney shape. The chimneys pass through the external covering of the furnace 7 and are closed by mobile, preferably heat insulated, covers 5 arranged side by side. The interstice between the fixed vand stationary bipolar electrodes 1, preferably comprising graphite, and the anodic layer l4 is lledup by a liquid lm 17 which may comprise molten aluminum. The layer 4 is of the selfrestoring type, kpreferably comprising electrode carbon, pre-baked or self-baking. The bipolar electrodes rest on ledges or'sockets 18, 19 and 20 of said refractory material. This segregates or separates the lower chambers that collect the aluminum produced in the contiguous cells, for instance 16 and 22. The lower, recess-free, chambers 11 are here reduced to minimum dimensions. The depth and Volume of chamber 11 are only minor or small portions of the corresponding dimensions of the electrode gap above it. The fraction may range from about a fourth Vto a tenth or less. There is no longer any need for a chamber of great capacity for collecting the aluminum, since the present invention permits operation with liquid layers not subjected to sudden level variations, such as are caused, for example, by periodic tapping. 'This surprising result is due to constancy of supply of all of the starting materials partaking in the production cycle (A1203 and auodes), and to constancy in removal of all of the products of electrolysis, the aluminum and gas evolved at the electrodes, although keeping the lower chambers and the metal therein electrically insulated, or at least not short circuited between each other. The lower chamber 11 of each cell is connected, through the conduit 12, with a corresponding individual, vertical, or nearly vertical, tapping well or pocket 28 (FIG. 4), at a suitable level of which a slit serving as an overow weir is provided.

In each cell the bath of the electrode gap 16, or 22, is in contact with the inclined, .upwardly facing cathode faces of the bipolar electrodes 1 and with the self-restoring anode assemblies '4. The electrode gap is limited laterally by the walls 23 (FIGS. 2. and 3) of the already mentioned refractory material. The fixed bipolar electrodes 1, the -self-restoring anodes 4 and the interposed liquid film 17, are subdivided by the refractory walls 13 (FIG. 2) so that in this example every cell actually comprises three anodes and three cathodes.

The restoring anodes 4 are visible in FIG. 2, in both sectionsof the furnace, taken at two levels, as explained above, while of the chimney-shaped piece 3 only the front-side submerged in the bath is visible.

The left half of FIG. 3, taken along line B-B of FIG. l, represents ahorizontal section of the cells taken at the level of the chambers for gas discharge, while the right half is a top View, as explained above. In this view 6 represents a top View ofthe fixed heat insulator arranged above the horizontal refractory block 2. Numerals 3 and 4 respectively designate the section of the chimney-shaped piece and the section of the restoring anode. At S is the cover ofthe chimney and 8 is the mobile heat-*insulating layer interposed between the free bath surface below and the gas-discharge chamber above. At 9 (FIGS. 1 and 3) is the supporting joist or hatten disposed under the mobile insulating layer.

FIG. 4 is a sectional view taken along line C-C of FIGS. l and 5. It is a cross section through two cells disposed yside by side and belonging to the two parallel branches of the above-described necklace-type furnace. The branches are separated by a longitudinal, dividing I wall structure in which is formed the longitudinal channel 29.

This figure illustrates how the chimney-shaped pieces 3 enclose the upper portions of each of the self-restoring anodes 4. In the lower portion of the cell is visible the bath layer 16 (cryolite for example) superimposed on the layer of molten metal 11 (aluminum). The metal covers the bottom 21 of the lower chamber. At 27 is a normally closed discharge at the bottom of the cell, directed towards the outside. The lower chamber for the metal is in communication, at the side .opposed to the discharge 27, through channel 12, with the already described tapping pocket 28, closed at the top by heat-insulating cover 25. Each pocket has a slit 24 which can act as an overflow or better overfall and puts the pocket in communication with the channel,` reservoir or receptacle 29 formed in the interior refractory dividing wall of the necklace furnace. The channel is closed by heat insulated covers 26. On the bottom of said channel, or receptacle, there is built, or formed, a'longitudinal partition 30, which however is optional, and which divides the receptacle into two longitudinal compartments designed to collect the aluminum coming from the overflow slits 24 of the two branches of the furnace. The height of said overflow slits above the Vbottom 21 of the lower aluminum collecting chamber should be so calculated that the static-pressures on bottom 21, and on the base of pocket 28 are equal. That is the weight of the column of molten aluminum bearing upon the unit area of the base of the tapping pocket 28 is equal to the counter-pressure exerted by the liquid column on the unit of area of the 'bottom 21 of the corresponding lower aluminum chamber. The latter liquid column is composed of aluminum in its lower portion and in its upper portion of bath, said aluminum and said bath having the same heights as the corresponding two liquid layers present in the lower chamber and in the electrode gap and the upper flared space (visible in FIG. 1) formed by the horizontal, refractory blocks 2 constituting the upper heads of the electrodes and the front walls of the chimney pieces. The height of the liquid column in the cell, is always higher than that in the tapping pocket, since the density of the bath is about 2.1 and that of the molten metal is about 2.3. The receptaclemay have at least sufficient capacity to accept all of the aluminum Vas currently produced in the cells connected thereto, during the period between two successive operations of removing from the furnace the metal thus separated.

The aluminum produced electrolytically flows down the cathode faces towards the cell bottom. This tends to raise the level 15 (FIGS. l and 4) of the layer of metal in the lower chamber. In such case the weight of the liquid column per unit area existing in the cell will prevail over the weight of the liquid column per unit area in the corresponding tapping pocket. Consequently there occurs a virtual increase of the height of the liquid level in the tapping pocket, so that said level surpasses the level of the overflow Weir or slit 24. yAs soon as a quantity of aluminum equal to the quantity produced by electrolysis in the cell is discharged through said overflow into the receptacle 29, equilibrium conditions are restored between the cell and the pocket. Thus constancy of the liquid levels vis automatically ensured in .the individual cells, of both the bath and aluminum. This is a dynamic equilibrium, since constancy of the height of level of the bath in the cell is in practice-ensured by the regular ow of the bath liquid through all the cells connected in closed.

circuit, while on lthe contrary the height and constancy of the levelfof metal in the cell are ensured by the aforesaid overflow weir, as well as by the constancy of bath level.

In FIG. 4 the aluminum that overows falls down preferably in form of separate drops and collects at the bottom of the twolongitudinal compartments of the receptacle is rfurnace and not shown in the drawing, sible to lift and to lower the electrodes, for example by the amount needed for compensating,

Fconstant, in practice. `control by any known automatic adjusting device (not indicated at 31. Cell lining 23 may be made of refractory material based on MgO, silicon carbide, etc.

FIG. 5, taken along line D-D of FIG. 4, is a longitudinal section of the same closed circuit, multicell furnace. It is taken near the center line of the furnace, and thus longitudinally sections the receptacle or channel 29 employed for collecting the molten aluminum produced. The aluminum is removed from said receptacle when desired, for further processing by known methods. The conduits 32 and 35 connect the two parallel branches of the necklace-type multicell furnace to ensure the circulation of the bath in a closed circuit enclosed Within the furnace. The circulation is carried out with the aid of a device for lifting the bath not shown in the drawing, for example of the kind described in the application of G. Calabria, Serial No. 670,785, led Iuly 9, 1957, now U.S. Patent No. 2,991,240, describing an oscillating dipping bucket raising molten bath from one terminal chamber to a second, the rst chamber having a lower bottom wall than the second and serving to receive molten bath liquid from a longitudinally disposed battery of cells, the bath liquid recirculating from the first terminal chamber through another longitudinally disposed battery of cells and back to the other battery. The lifting may also be accomplished by means of gas pressure as in the application of G. de Varda No. 587,985, led May 29, 1956, or by means of the linearly reciprocating, valveless graphite piston described in A. V. de Pava application 'Serial No. 705,373, filed December 26, 1957, now U.S. 'Patent No. 3,063,930 or by other known devices. The valuminum 31 and 34 falling from the weirs of individual tapping pockets collects on the bottom wall of one of the two longitudinal compartments formed by Wall 30. The aluminum forms one or more deposits at the lowest points of said bottom wall. The longitudinal compartment may be divided by one or more transverse partitions 33 (FIG. to keep separate the product of individual groups of contiguous cells, if necessary, in view of the grade of the metal to be produced. The transverse partitions, as well as the longitudinal partition 30, may have horizontal bottoms or bottoms of opposed inclinations. The partitions are useful but may be omitted. On the other hand, if it is desired to prevent the product of some of the cells from becoming mixed with that of the furnace as a whole, or with that of a group of contiguous cells, it is always possible to remove the aluminum from the selected cell, or cells, by aspiration, by known means, through the corresponding tapping pockets, after having removed the respective upper plug. In such case the n 'level of the metal descends in the pocket below the level of the respective overflow, to return to the initial level only after the production, in the corresponding cell, of a quantity of metal equivalent to that removed directly from 'the pocket. The covers 26 of the receptacle 29 are opened only when the operation of removing the metal -produced is carried out.

The process and the device for continuous tapping both of which form part of the present invention, may also be submerged in a layer of molten bath 140. Electrodes 40 are separated, by the electrode spacing 50, from the unrderlying layer of molten aluminum 60 produced electro- 'lytica1ly. The electrodes are controlled by known mechanical devices, the control devices being outside the whence it is posprogressively, for the consumption by electrolysis of carbon anode 40, and of the bath layer, thus keeping the electrode spacing 50 It is preferable to replace hand .the level and the balance lThe vertical walls 131, 150 etc. contacting the receptacles Vof by a refractory lining, ,the furnace providing `that of the other figures, is 'provided with refractory walls 131 made preferably of silicon nitride bonded silicon carbide (refrax) or of magnesium oxide which have been rendered impermeable to the bath by an inert gas current as described in the application of G. Calabria Serial No. 706,381, tiled December 31, 1957, now abandoned or by impregnating with a solution of pitch, removing excess surface pitch, and therefore carbonizing, as described in my application Serial No. 705,374, filed December 26, 1957, now U.S. Patent No. 2,952,605. But any conventional material, for instance carbonaceous masses or blocks may be used too.

The thin walls 70 and 90 (FIG. 6) form an interspace which communicates, through holes provided at the base of the wall 70, with the bottom layer 60 of cathodic metal and, by means of the overflow weirs 100, with the receptacle 120 preferably provided in the pot zone intermediate and between the two self-baking electrodes 40. i

The interspace formed by the walls 70 and 90 is closed by the cover 101, and the receptacle 120 by the covers 200.

The apparatus is thus arranged so that the liquid 300 (FIG. 6) penetrating into the interspace is formed exclusively of molten aluminum, and not of bath. If the level of the free surface of the bath is kept constant, the height of the overflow Weir determines the height or level of the metal in the zone under the electrodes. To keep the bath level constant it is necessary to supply the bath with alumina in a sufficiently continuous manner, and to keep, as already stated, the height of the lower base planes of the two electrodes substantially constant. Under these conditions, the aluminum produced by electrolysis will cause an equivalent quantity of aluminum to overflow from the interspace into the receptacle 120, on the bottom of which the layer of aluminum 111 forms. This deposit of aluminum can be removed for subsequent known processing steps, being taken, when needed, without interfering to the least extent with the constancy of in or of the electrolysis operation in the cell proper. The deposit is removed by lifting the cover 200 and sucking out by a known technique, for instance by means` of a mobile pipe submerged from above, allor part of the aluminum 111.

FIG. 7 is a horizontal section of the same furnace also illustrates atop view ofthe cathodic bottom part.

bath and the molten aluminum are of the already mentioned refractory inert material. The continuous feeding of alumina is carried out according to known methods, the

alumina being preferably fed into zone 160 disposed internally with respect to the head walls 150 of the two indicated in the drawing.

I claim:

1. A furnace for producing aluminum by'electrolyss alumina in a molten salt bath, comprising a containing furnace wall structure at least in part lined internally lanode and cathode means therein, a lower chamber therein for receiving the aluminum produced, means in the furnace providing a refractory lined tapping pocket forY aluminum metal and a passage connecting the lower chamber with the lower portion of said pocket for ow of aluminum metal, a refractory lined hot receptacle for aluminum in the furnace, and an over-fall device by which the receptacle receives molten aluminum from said lower chamber, the body of molten aluminum in the receptacle having a free upper surface below the over-fall device,

so that the molten aluminum is removable at will from the receptacle Without disturbing the level of molten aluminum in the lower chambers, the height of the overthe height of theoverflow weir fall device being such that the static liquid pressures on the bottom of the lower chamber and at the bottom of the tapping pocket are substantially equal, said receptacle being otherwise isolated from said bath, the tapping pocket and the receptacle being located within and inwardly removed from the refractory outer side walls of said furnace, and being in proximity to the molten salt bath. a

2. A furnace for producing aluminum by electrolysis of alumina in a molten salt bath, comprising a containing refractory furnace wall structure, anode and cathode means therein, the furnace providing a lower chamber for receiving the aluminum produced, means in the furnace providing a Vhot storage receptacle for aluminum and providing an overflow weir over which the molten aluminum flows down into the receptacle, said molten aluminum coming from said lower chamber, an upwardly-downwardly directed tapping pocket for the aluminum, the pocket communicating at its lower end portion with the said lower chamber, for removal of molten aluminum therefrom, and communicating, by means of the Weir, with the receptacle for flow of aluminum to the latter, being such that Ithe static liquid pressures on the bottom of the lower chamber and at the bottom of the tapping pocket are substantially equal, said receptacle being otherwise isolated from said bath, the tapping pocket and the receptacle being located within and inwardly removed from the refractory outer side walls of said furnace, and being in proximity to the molten salt bath.

3. A furnace for producing aluminum by electrolysis of alumina in a molten salt bath, comprising a containing refractory furnace wall structure, a plurality of anode and cathode means therein providing a plurality of cells, said furnace providing passages for serial flowand serial circulation of the molten bath amongst the cells, the passages including over-ow conduits for determining the bath level in the cells, the furnace providing a lower chamber for receiving the aluminum produced, means in the furnace providing a hot storage receptacle for aluminum and providing an overilow Weir over which the molten aluminum flows down into the receptacle, said molten aluminum coming from said lower chamber, an

Aupwardly-downwardly directed tapping pocket for the aluminum, the pocket communicating at its lower end `portion with the said lower chamber, for removal of molten aluminum therefrom, and communicating, by means of the Weir, with the receptacle for flow of aluminum to the latter, the height of the weir being such that the static liquid pressures on the bottom of the lower chamber and at the bottom of the tapping pocket are substantially equal, said receptacle being located Within the structure of the furnace in proximity to the molten salt -bath and being between, and receiving aluminum from, adjacent cells.

4. A heat insulated, refractory lined furnace for producing aluminum by electrolysis of alumina in a fused fluorinated bath, comprising therein two groups of cells, k

an intermediate refractory wall separating the groups, the wallproviding therein receptacle means for the aluminum produced, the groups each comprising upwardly-downwardly extending electrode structures providing opposed pairs of anodic and cathodic surfaces, each pair defining therebetween an upwardly-downwardly extending electrolysis gap, the furnace providing a bottom wall and structure for collection of an individual molten aluminum layer below the upper bath layer of each cell, upwardly extending tapping pockets communicating below with the individual aluminum layers on the bottom wall for Yreception of aluminum therefrom, weir means over which the aluminum flows from the pockets and falls down into the receptacle means, so that molten aluminum is removable at will from the receptacle means without disturbing the level of the molten aluminum on the bottom wall of each cell, said furnace providing over-flow con- -duits `serially connecting 'the electrolysis gaps of the cells .to determine the maximum height of the bath in said gaps.

5. A heat insulated, refractory lined furnace for producing aluminum by electrolysis of alumina in a fused fluorinated bath, comprising therein two groups of cells, an intermediate refractory wall separating the groups, the wall providing therein receptacle means for the aluminum produced, the groups each comprising upwardlydownwardly extending electrode structures providing opposed pairs of anodic and cathodic surfaces, each pair defining therebetween an upwardly-downwardly extending electrolysis gap, the furnace providing a bottom wall for collection of a molten aluminum layer-below the upper bath layer, upwardly extending tapping pockets cornmunicating below with the layer on the bottom wall for reception of aluminum therefrom, each electrolysis gap having an individual tapping pocket, the lower portions of the gaps being isolated from communication with each other to ensure separate removal of the aluminum Vto the respective tapping pocket, weir means over which the aluminum flows from the pocket down into the receptacle means, the effective height of the overflow weir being such that the static pressure of the bath layer andmetal layer on said bottom wall .is substantially equal to the static pressure in the respective tapping pocket at the level thereof corresponding to the said bottom wall.

6. A heat insulated, refractory lined furnace .for producing aluminum by electrolysis of alumina in a fused fluorinated bath, comprising therein two groups of cells, an intermediate refractory wall separating the groups, the wall providing therein receptacle means for the aluminum produced, the groups each comprising upwardlydownwardly extending electrode structures providing opposed pairs of anodic and cathodic surfaces, each pair defining therebetween an upwardly-downwardly extending electrolysis gap, the furnace providing a bottom wall for collection of a molten aluminum layer below the upper bath layer, upwardly extending tapping pockets communieating below with the layer on the bottom wall for reception of aluminum therefrom, each electrolysis gap having an individual tapping pocket, the lower portions o'f the gaps being isolated by transversal partitions of refractory material having poor electric conductivity from communication with each other to avoid by-passage of electric current and to ensure separate removal of the aluminum to the respective tapping pocket, weir means over which the aluminum flows from the pockets down into the receptacle means.

7. A heat insulated, refractory lined furnace for prov ducing aluminum by electrolysis `of alumina, whose content, in a fused iiuorinated bath is kept practicallyconstant, comprising therein two groups of cells, an intermediate refractory wall separating ythe groups, the wall providing therein receptacle means for the aluminum produced, the groups each comprising upwardly-downwardly extending bipolar intermediate electrode structures each cell, upwardly extending tapping pockets communicating below with the bottom wall for reception of aluminum therefrom, each electrolysis gap having an individual tapping pocket, the lower portions of the gaps being isolated from communication with each other for separate removal of the aluminum to the respective tapping pocket, weir means over which the aluminum flows from the pockets to the receptacle means, whereby continuous anode surface restoration, controlled removal of aluminum from the gaps to maintain the liquid levels constant in the cells, and substantially constant spacing between the anodic and cathodic surfaces is maintained.

8. A heat insulated, refractory lined furnace for producing aluminum by electrolysis of alumina in a fused fluorinated bath, comprising therein two groups of cells, an intermediate refractory wall separating the groups, the walls providing therein receptacle means for the aluminum produced, the groups each comprising upwardly-downwardly extending electrode structures providing opposed pairs of anodic and cathodic surfaces, each pair defining therebetween an upwardly-downwardly extending electrolysis gap, the furnace providing a bottom wall for collection of a molten aluminum layer below the upper bath layer, upwardly extending tapping pockets communicating below with the layer on the bottom Wall for reception of aluminum therefrom, each electrolysis gap having an individual tapping pocket, the lower portions of the gaps being isolated from communication with each other to permit separate removal of the aluminum to the respective tapping pocket, Weir means over which the aluminum flows from the pockets down into the receptacle means, the receptacle means comprising an elongated channel having a bottom wall, the weir means being formed in opposite longitudinally directed walls of the channel, a longitudinal overflow partition in the channel on the bottom wall, dividing it into two troughs, at least one transverse overow partition intermediate the length of the channel to form several aluminum pools on the bottom wall, and means for recirculating the bath among the cells in a closed circuit within the furnace.

9. A multicell furnace for production of molten aluminum by electrolysis of alumina in a molten salt bath, comprising a series of upwardly-downwardly extending, longitudinally spaced bipolar electrode structures each having opposite walls providing anodic and cathodically active surfaces, the anodic surfaces comprising self-restoring carbonaceous material consumable in the electrolysis, the opposed active surfaces of each pair of contiguous bipolar electrodes remaining at a constant distance, delining between them upwardly-downwardly extending electrolysis gaps, the bottom of the furnace having a refractory, inert, impervious lining, the lining being formed to provide upwardly extending ledges upon which the bipolar electrodes rest, the ledges serving to separate and isolate the lower parts of the adjacent electrolysis gaps from each other, the ledges and the parts of the respective bipolar electrode resting thereon being of substantially the same longitudinal dimension, the ledges providing aluminum collecting spaces therebetween, the said spaces each having a height and a volume each of which is not more than about a fourth of the respecive electrolysis gap, and tapping pocket means for conducting aluminum from the spaces, and an overfiow Weir and receptacle means to receive the overflowing aluminum, the pocket, weir, and receptacle means being in the furnace in heat receiving relation with respect to the molten bath, the molten aluminum dropping down over the Weir into the receptacle means, so that the molten aluminum is removable at will from the receptacle without disturbing the level of the molten aluminum in said spaces, said furnace providing passages for serial flow and serial circulation of molten bath amongst the cells, the height of the weir being such that the static liquid pressures on the bottoms of the said spaces and at the bottom of the tapping pocket means are substantially equal, said passages including over-flow conduits connecting the electrolysis gaps of the cells to determine the maximum height of the bath in said gaps.

lO. A furnace for producing aluminum by electrolysis of alumina in a molten salt bath, comprising a containing furnace wall structure at least in part lined internally by a refractory lining, a plurality of anode and cathode means therein providing a plurality of cells, the furnace providing passages for serial flow and re-circulation of bath liquid amongst the cells and to determine the liquid level of the bath in the cells during operation, and providing kindividual lower chambers therein for receiving the aluminum produced by each cell, means in the furnace providing a hot receptacle for aluminum and providing overflow means through which the receptacle receives molten aluminum from said lower chambers, the body of molten aluminum in the receptacle having a free upper surface below the overflow means, the molten aluminum dropping down into and being caught in the receptacle, and being removable at will from the receptacle without disturbing the level of molten aluminum in the lower chamber, said receptacle being lo- .cated in heat exchange relation with the molten bath, said receptacle being located within the refractory structure of the furnace in proximity to the molten salt bath and being between, and receiving aluminum from, adjacent cells thereof.

11.1A furnace for producing aluminum by electrolysis of alumina in a molten salt bath, comprising a containing refractory furnace wall structure, anode and cathode means therein providing a cell, the furnace providing passages for re-circulation of bath liquid from and to the cell and to determine the liquid level of the bath in the cell during operation, the furnace providing a lower chamber for receiving aluminum produced, means in the furnace providing a hot storage receptacle for aluminum and providing an overflow weir over which the molten aluminum drops down into thevreceptacle, said molten 'aluminum coming from said lower chamber, said receptacle being located in heat exchange relation with the molten bath.

l2. A furnace for producing aluminum by electrolysis of alumina in a molten salt bath, comprising a containing refractory furnace wall structure, anode and cathode means therein, the furnace providing a lower chamber for receiving the aluminum produced, means in the furnace providing a hot storage receptacle for aluminum and providing an over-flow weir over which the molten aluminum drops down into and is caught in the receptacle, the body of molten aluminum in the receptacle having a free upper surface below the over-flow Weir, said molten aluminum coming from said lower chamber, an upwardlydownwardly directed tapping pocket for the aluminum, the pocket communicating at its lower end portion with the said lower chamber, for removal of molten aluminum therefrom, and communicating, by means of the weir, with the receptacle for flow of aluminum to the latter, said receptacle being located in heat exchange relation with the molten ba 13. A furnace for producing aluminum by electrolysis of alumina in a molten salt bath, comprising a containing refractory furnace wall structure, anode and cathode means therein, the furnace providing a lower chamber for receiving the aluminum produced, means in the furnace providing a hot storage receptacle for aluminum and providing an overflow weir over which the molten aluminum drops down into and is caught in the receptacle, the body of molten aluminum in the receptacle having a free upper surface below the over-flow Weir, said molten aluminum coming from said lower chamber, an upwardly-downwardly directed tapping pocket for the aluminum, the pocket communicating at its lower end portion with the said lower chamber, for removal of molten aluminum therefrom, and communicating, by means of the weir, with the receptacle for flow of aluminum to the latter, the height of the weir being such that the static liquid pressures on the bottom of the lower chamber and at the bottom of the tapping pocket are substantially equal, said receptacle being located in heat exchange relation with the molten bath.

14. A heat insulated, refractory lined furnace for producing a metal by electrolysis of a compound of the metal in a molten salt bath, comprising a plurality of opposed anode and cathode electrodes defining between them a plurality of upwardly-downwardly directed serially disposed electrolysis gaps, the furnace providing passages therewithin for serial flow and re-circulation of bath liquid amongst the cells and for determining the bath level in the cells, means providing a bottom portion for collecting metal, produced in the process, below an upper layer of bath, means Within the refractory forming an upwardly extending tapping well and a passage permitting flow of the metal to the well as it collects on the bottom portion, a hot refractory receptacle in the furnace serving as a common reservoir for the metal produced in said electrolysis gaps, the receptacle extending in a direction transverse to the series of electrolysis gaps, an overow device, the metal passing from the bottom portion upwardly in the Well and then over the overtlow device and dropping down into the receptacle, the body of metal in the receptacle having a free upper surface below the over-flow device, the effective height of the overow device being such that the static pressure of the bath and metal layers on said bottom portion is substantially equal to the static pressure in the tapping well at the level thereof corresponding to the bottom of the bottom portion, the tapping Well and the receptacle opening upwardly, and removable lid means for the Well and receptacle, said receptacle being located in heat exchange relation with the molten bath, said receptacle being in proximity to the molten salt bath and located between, and receiving aluminum from, adjacent cells 15. In a furnace process of producing aluminum by electrolysis of an aluminum compound in a fused bath not subject to periodical variations of its composition,

in a cell operated at a practically constant interelectrodic spacing and comprising an anodic carbonaceous active surfacetconsumable in the process, vthe molten aluminum produced forming a bottom layer beneath the bath layer, the height of each of said two layers `being maintained substantially constant, the bath being re-circulated Within the furnace from and lto the cell, .the improvement comprising removing upwardly substantially continuously, a column of aluminum from the bottom layer, and permitting the top of .the column to overilow and fall downwardly, and collecting the downward overilow in a hot storage chamber, the body of aluminum in the storage chamber having a free upper surface below the top of the column, said column of aluminum and the downward overow being in heat exchange relation with the molten bath.

References Cited in the tile of this patent UNITED STATES PATENTS 995,476 McNitt June 20, 1911 v2,480,474 Johnson Aug. 30, 1949 2,902,415 Niedrach et al Sept. 1, 1959 2,952,592 De Varda Sept. 13, 1960 FOREIGN PATENTS 484,197 Germany Oct. 10, 1929 254,311 Switzerland.- Dec. 16, 1948 451,182 Italy Aug. 27, 1949 1,119,832 France Apr. 9, 1956 

1. A FURNACE FOR PRODUCING ALUMINUM BY ELECTROLYSIS OF ALUMINA IN A MOLTEN SALT BATH, COMPRISING A CONTAINING FURNACE WALL STRUCTURE AT LEAST IN PART LINED INTERNALLY BY A REFRACTORY LINING, ANODE AND CATHODE MEANS THEREIN, THE FURNACE PROVIDING A LOWER CHAMBER THEREIN FOR RECEIVING THE ALUMINUM PRODUCED, MEANS IN THE FURNACE PROVIDING A REFRACTORY LINED TAPPING POCKET FOR ALUMINUM METAL AND A PASSAGE CONNECTING THE LOWER CHAMBER WITH THE LOWER PORTION OF SAID POCKET FOR FLOW OF ALUMINUM METAL, A REFRACTORY LINED HOT RECEPTACLE FOR ALUMINUM IN THE FURNACE, AND AN OVER-FALL DEVICE BY WHICH THE RECEPTACLE RECEIVES MOLTEN ALUMINUM IN THE RECEPTACLE HAVING A FREE UPPER SURFACE BELOW THE OVER-FALL DEVICE, SO THAT THE MOLTEN ALUMINUM IS REMOVABLE AT WILL FROM THE RECEPTACLE WITHOUT DISTURBING THE LEVEL OF MOLTEN ALUMINUM IN THE LOWER CHAMBERS, THE HEIGHT OF THE OVERFALL DEVICE BEING SUCH THAT THE STATIC LIQUID PRESSURES ON THE BOTTOM OF THE LOWER CHAMBER AND AT THE BOTTOM OF THE TAPPING POCKET ARE SUBSTANTIALLY EQUAL, SAID RECEPTACLE BEING OTHERWISE ISOLATED FROM SAID BATH, THE TAPPING POCKET AND THE RECEPTACLE BEING LOCATED WITHIN AND INWARDLY REMOVED FROM THE REFRACTORY OUTER SIDE WALLS OF SAID FURNACE, AND BEING IN PROXIMITY TO THE MOLTEN SALT BATH. 